How can the idea of microcosm and macrocosm be applied to Hamlet and Macbeth?
A "microcosm" is an individual or community considered as a seperate universe; "macrocosm" is the entire great world as a whole.
In both of Shakespeare's plays the element of subplot is introduced (microcosm) as part of the whole (macrocosm).
Some examples in Macbeth are each individual existence of the noble families--the House of Fife, the House of Glamis, the House of Cawdor. These are microcosms within the play since each of these families along with their families live in seclusion from the rest of Scotland in their own, self-sufficient lands. Scotland, itself, is a microcosm in the play when you consider that Macduff and others have to seek aide outside of those borders to save her from Macbeth. England, then, would be a greater part of the universe, and representative of macrocosm.
You could also consider the world of the witches, Hecate and all, as a microcosm. Existing entirely isolated from the world of the humans until they wish to interfere and stir up trouble.
In Hamlet, you also have a microcosm of the paranormal--King Hamlet's ghost as well as Banquo's ghost who exist in a world divided from the whole of humanity. There is the the microcosm of Denmark and the back story of the war which is wholly divided from the picture of events ocurring in Hamlet's castle and world. In this play, there is also the world of the actors and the world they create which can be considered a microcosm. Hamlet also sets himself apart from the rest of the macrocosm within psychological self during his decision-making process.
These are just some jumping-off places. Maybe you will be able to come up with other examples! Good Luck!